The present invention relates to a current steering type D/A (digital-to-analog) converter to be used in a semiconductor integrated circuit and, more particularly, to an improved current steering type D/A converter containing plural circuit blocks.
As an LSI grows in performance, there is an increasing need for a current steering type D/A converter mounted on the LSI chip to change its operation mode according to the use of the LSI, for example, to operate internal D/A conversion circuits in their normal states or power down states according to the use mode of the LSI.
FIG. 5 is a block diagram illustrating a conventional current steering type D/A converter 500.
With reference to FIG. 5, the current steering type D/A converter 500 comprises a D/A conversion circuit 510, a reference voltage generation circuit 520, and a power down control circuit 530. The D/A conversion circuit 510 comprises plural current sources 5101 each outputting a predetermined current, and a logic circuit 5102 for selecting required current sources corresponding to a digital input code, from among the plural current sources 5101. The reference voltage generation circuit 520 outputs a reference voltage VB, and this reference voltage VB determines a current value IOUT to be outputted from a current output terminal DAOUT of the D/A conversion circuit 510. A P channel MOS transistor constituting the power down control circuit 530 performs a switching operation for switching the current steering type D/A converter 500 between its normal state and power down state, according to a power down control signal supplied from a power down control terminal NPD.
Hereinafter, the operation of the current steering type D/A converter 500 constituted as described above will be described for its normal state and power down state.
First of all, when a xe2x80x9cLowxe2x80x9d signal (hereinafter referred to as a xe2x80x9cLxe2x80x9d signal) is inputted as a power down control signal to the power down control terminal NPD, the current steering type D/A converter 500 goes into the power down state. That is, the xe2x80x9cLxe2x80x9d signal inputted as the power down control signal brings the power down control circuit 530 into conduction, and a potential at a fixed potential node 560 is applied to a wiring 550 which connects a reference voltage VB node of the D/A conversion circuit 510 and a reference voltage node of the reference voltage generation circuit 520. The fixed potential node 560 has previously been set at a potential that halts the current sources 5101 of the D/A conversion circuit 510 and the reference voltage generation circuit 520. When this potential is applied to the D/A conversion circuit 510 and the reference voltage generation circuit 520, the current sources 5101 of the D/A conversion circuit 510 and the reference voltage generation circuit 520 halt the operations.
Next, when a xe2x80x9cHighxe2x80x9d signal (hereinafter referred to as a xe2x80x9cHxe2x80x9d signal) is inputted as a power down control signal to the power down control terminal NPD, the current steering type D/A converter 500 operates in the normal state. That is, the xe2x80x9cHxe2x80x9d signal inputted as the power down control signal brings the power down control circuit 530 into the non-conducting state, and the potential of the fixed potential node 560 is not applied to the wiring 550. The reference voltage generation circuit 520 contains a power on control circuit (not shown) and a fixed potential node (not shown) corresponding to the power down control circuit 530 and the fixed potential node 560, respectively, and the fixed potential node (not shown) is set at a voltage that operates the reference voltage generation circuit 520. When the power on control circuit (not shown) receives the xe2x80x9cHxe2x80x9d signal, it transfers the voltage of the fixed potential node (not shown) to the main unit of the reference voltage generation circuit 520. On receipt of the xe2x80x9cHxe2x80x9d signal as the power down control signal, the reference voltage generation circuit 520 outputs a reference voltage VB, and this voltage is supplied to the current sources 5101 of the D/A conversion circuit 510. Subsequently, the logic circuit 5102 selects current sources corresponding to a digital input code, from among the plural current sources 5101, and a current IOUT that is the sum of the currents of the selected current sources is outputted from the current output terminal DAOUT.
As described above, in the case where the single D/A conversion circuit 510 is provided corresponding to the single reference voltage generation circuit 520, the MOS transistor 530 functioning as the power down control circuit switches the supply of power to the D/A conversion circuit 510 so that the D/A conversion circuit 510 goes into the normal state or the power down state.
Next, a description will be given of a conventional current steering type D/A converter in which plural D/A conversion circuits are provided corresponding to a single reference voltage generation circuit.
FIG. 6 is a block diagram illustrating a conventional current steering type D/A converter 600 having plural D/A conversion circuits. In FIG. 6, only two D/A conversion circuits are shown for the sake of simplicity.
With reference to FIG. 6, the current steering type D/A converter 600 comprises a first D/A conversion circuit 611, a second D/A conversion circuit 612, a reference voltage generation circuit 620, and a power down control circuit 630.
The first D/A conversion circuit 611 comprises plural current sources 6111 each outputting a predetermined current, and a first logic circuit 6112 for selecting required current sources corresponding to a digital input code, from among the plural current sources 6111. The second D/A conversion circuit 612 comprises plural current sources 6121 each outputting a predetermined current, and a second logic circuit 6122 for selecting required current sources corresponding to a digital input code, from among the plural current sources 6121. The reference voltage generation circuit 620 outputs a reference voltage VB, and the reference voltage VB determines a first current value IOUT1 to be outputted from a first current output terminal DAOUT1 of the first D/A conversion circuit 611, as well as a second current value IOUT2 to be outputted from a second current output terminal DAOUT2 of the second D/A conversion circuit 612. A P channel MOS transistor constituting the power down control circuit 630 performs a switching operation for switching the current steering type D/A converter 600 between its normal state and power down state, according to a power down control signal supplied from a power down control terminal NPD.
Hereinafter, the operation of the current steering type D/A converter 600 constituted as described above will be described for its normal state and power down state.
First of all, when a xe2x80x9cLxe2x80x9d signal is inputted to the power down control terminal NPD as a power down control signal, the current steering type D/A converter 600 operates in the power down state. That is, the xe2x80x9cLxe2x80x9d signal inputted as the power down control signal brings the power down control circuit 630 into conduction, and a potential of a fixed potential node 660 is supplied to a wiring 650 which connects the reference voltage VB nodes of the D/A conversion circuits 611 and 612 and the reference voltage VB node of the reference voltage generation circuit 620. The fixed potential node 660 has previously been set at a potential that halts the current sources 6111 and 6121 of the D/A conversion circuits 611 and 612 and the reference voltage generation circuit 620. When this potential is applied to the first and second D/A conversion circuits 611 and 612 and the reference voltage generation circuit 620, the respective current sources 6111 and 6121 of the D/A conversion circuits 611 and 612 and the reference voltage generation circuit 620 halt the operations.
On the other hand, when a xe2x80x9cHxe2x80x9d signal is inputted to the power down control terminal NPD as a power down control signal, the current steering type D/A converter 600 operates in the normal state. That is, the xe2x80x9cHxe2x80x9d signal inputted as the power down control signal brings the power down controller 630 into the non-conducting state, and the potential of the fixed potential node 660 is not supplied to the wiring 650. The reference voltage generation circuit 620 contains a power on control circuit (not shown) and a fixed potential node (not shown) which correspond to the power down control circuit 630 and the fixed potential node 660, respectively, and the fixed potential node (not shown) is set to a potential that operates the reference voltage generation circuit 620. When the power on control circuit (not shown) receives the xe2x80x9cHxe2x80x9d signal, it operates the reference voltage generation circuit 620 with the potential of the fixed potential node (not shown). The reference voltage generation circuit 620 outputs a reference voltage VB, and this voltage is supplied to the plural current sources 6111 and 6121 of the D/A conversion circuits 611 and 612. Subsequently, the first logic circuit 6112 selects current sources corresponding to a digital input code from among the plural current sources 6111, and a current IOUT1 that is the sum of the currents of the selected current sources is outputted from the first current output terminal DAOUT1. Likewise, the second logic circuit 6122 selects current sources corresponding to a digital input code from among the plural current sources 6121, and a current IOUT2 that is the sum of the currents of the selected current sources is outputted from the second current output terminal DAOUT2.
As described above, in the case where the plural D/A conversion circuits 611 and 612 are provided corresponding to the single reference voltage generation circuit 620, the MOS transistor 630 functioning as the power down control circuit switches the supply of power to the D/A conversion circuit 611 (612) so that the D/A conversion circuit 611 (612) goes into the normal state or the power down state.
To be specific, in the conventional current steering type D/A converter, the operation of the whole circuit is switched between the normal state and the power down state by inputting the xe2x80x9cHxe2x80x9d or xe2x80x9cLxe2x80x9d signal as the power down control signal to the power down control terminal NPD.
In the conventional current steering type D/A converter, however, when it is in the power down state, the reference voltage to be supplied to all of the D/A conversion circuits is fixed at a potential that halts the current sources of the D/A conversion circuits, whereby the flows of current in the respective D/A conversion circuits are stopped at the same time. Therefore, the respective D/A conversion circuits cannot be power-down-controlled individually.
That is, in the case where the conventional current steering type D/A converter 600 is provided with the plural D/A conversion circuits 611 and 612 corresponding to the single reference voltage generation circuit 620 as shown in FIG. 6, when a xe2x80x9cHxe2x80x9d signal is inputted to the power down control circuit 630 as the power down control signal, all of the reference voltage generation circuit 620 and the D/A conversion circuits 611 and 612 go into the conducting states at the same time. On the other hand, when a xe2x80x9cLxe2x80x9d signal is inputted to the power down control circuit 630 as the power control signal, all of the reference voltage generation circuit 620 and the D/A conversion circuits 611 and 612 go into the non-conducting states at the same time.
Accordingly, when the conventional current steering type D/A converter is provided with plural D/A conversion circuits, the reference voltage generation circuit 620 cannot set a specific D/A conversion circuit into its conducting state while setting the other D/A conversion circuit into its non-conducting states, that is, the reference voltage generation circuit 620 cannot set the respective D/A conversion circuits into the power down states individually.
The present invention is made to solve the above-described problems and has for its object to provide a current steering type D/A converter which can perform power down control for plural D/A conversion circuits individually.
Other objects and advantages of the invention will become apparent from the detailed description that follows. The detailed description and specific embodiments described are provided only for illustration since various additions and modifications within the scope of the invention will be apparent to those of skill in the art from the detailed description.
According to a first aspect of the present invention, there is provided a current steering type D/A converter comprising: a D/A conversion circuit for performing D/A conversion, comprising plural current sources each outputting a predetermined current, and a logic circuit for selecting desired current sources from among the plural current sources; a reference voltage generation circuit for generating a reference voltage potential which determines an output current value of the D/A conversion circuit; a first switch element having an end being connected to a reference voltage output terminal of the reference voltage generation circuit, and the other end being connected to a reference voltage input terminal of the D/A conversion circuit; a second switch element having an end being connected to a fixed potential for fixing the reference voltage potential at a predetermined potential that prevents current from flowing in the D/A conversion circuit, and the other end being connected to a node which connects the end of the first switch element and the reference voltage output terminal of the reference voltage generation circuit; and a control circuit for controlling the switch elements so as to set the first switch element into the ON state, and the second and third switch elements into the OFF states, when the D/A conversion circuit is to be operated in the normal state, and controlling the switch elements so as to set the first switch element into the OFF state, and the second and third switch elements into the ON states, when the D/A conversion circuit is to be operated in the power down state. Therefore, the D/A conversion circuit and the reference voltage generation circuit can be power-down-controlled individually, whereby current consumption of the current steering type D/A converter can be reduced.
According to a second aspect of the present invention, the current steering type D/A converter according to the first aspect comprises: n pieces of the D/A conversion circuits (n: integer equal to or larger than 2); the first switch element and the second switch element being provided for each of the n pieces of D/A conversion circuits; and, instead of the above-mentioned control circuit, a control circuit for controlling the switch elements so as to set the third switch element into the ON state, the n pieces of first switch elements into the OFF states, and the n pieces of second switch elements into the ON states, when all of the n pieces of D/A conversion circuits are to be operated in the normal states, controlling the switch elements so as to set the third switch element into the ON state, the n pieces of first switch elements into the OFF states, and the n pieces of second switch elements into the ON states, when all of the n pieces of D/A conversion circuits are to be operated in the power down states, and controlling the switch elements such that, when at least one of the n pieces of D/A conversion circuits is to be operated in the power down state while the remaining circuits are to be operated in the normal states, the third switch element is set into the OFF state, the first switch element and the second switch element of the D/A conversion circuit to be power-down-operated are set into the OFF state and the ON state, respectively, and the first switch elements and the second switch elements of the remaining circuits to be operated in the normal states are set into the ON states and the OFF states, respectively. Therefore, even when the current steering type D/A converter is provided with plural D/A conversion circuits, the respective D/A conversion circuits and the reference voltage generation circuit can be power-down-controlled individually, whereby current consumption of the current steering type D/A converter can be reduced by an amount of current to be consumed by the unused D/A conversion circuits.
According to a third aspect of the present invention, the current steering type D/A converter according to the first or second aspect further comprises a capacity element which is connected between the node connecting the reference voltage output terminal of the reference voltage generation circuit with the end of the third switch element, and the power supply potential or ground potential. Therefore, a low pass filter can be formed by the capacity element and the on-resistance components of the switch elements in the ON states, whereby noise of a high frequency component such as a sampling clock of the current steering type D/A converter is reduced, resulting in improved characteristics of the current steering type D/A converter.